Essentially, none of the sound samples (mp3) will play on duolingo.com. They do using Chrome, and they do using Vivaldi on Win 7. I'm using Mint MATE 18.3 64 bit, and sounds works perfectly everywhere else. Youtube and Vimeo etc, work properly under Vivaldi for Linux too. I've installed the widevine plug in at the Vivaldi mod's suggestion - no go, and my ffmpeg codecs are already the latest versions.